I'm not sure what deterrent effect posting a security sign would have, but there was a string of burglaries in my suburban neighborhood a few years ago, and every house that was broken into had an ADT or Broadview sign out in front (about half of them do around here).
Each entry was made through the front door, some of which were unlocked (geez, I'm guessing that they didn't actually bother with the security system either
), and the others were kicked in easily enough. None of these incidents were home invasions (i.e. with residents present), thank goodness, but more recently there was an attempted home invasion just a couple of houses away from mine, which was thwarted by the owner; it seemed that the would-be burglar had thought that the house was deserted and took off when he discovered otherwise. Yes, that house has a security sign, too.
While I have a minimal system that I installed myself, it's not monitored remotely by a security company, and I don't have any signage whatsoever in my front yard. A few months after the aforementioned string of burglaries, somebody tried to kick my front door in at around 2:30 am, but gave up after a bunch of attempts. All of my external doors were reinforced, and while this wouldn't have kept out somebody who was determined to get in at any cost, in this case it prevented a home invasion that probably would have ended with a lot of frazzled nerves and a dead or seriously injured burglar. It's hard to describe the layout, but I was waiting for him right by the door (I sleep lightly and my bedroom is not far from the door) and would have been able to attack him from behind had he successfully entered (he was alone, as far as I could see, but these guys do often operate in groups). I didn't own a gun at the time, but in all honesty I have great bat speed.
Since then, I've further hardened my home against invasions and upgraded my armament, of course.
Obviously, I or other family members could have been killed, but all I remember thinking about at the time was beating the burglar's brains out before he even knew what hit him. Fortunately, I didn't have to do anything that night. I guess I got this trait from my mom, who also gets extremely aggressive when she's threatened. Trying to keep a long story reasonably short, back in the "old country" she got fed up with taking physical and verbal abuse from an older brother of hers (her mother had passed away and her father was away on business), and one day when he smacked her around again and threatened to kill her because his life apparently sucked so much, without a word she attacked him with a chair and beat him unconscious. When he woke up he was at knifepoint and now on the receiving end of similar threats, so he moved out of the house, joined the army, was reportedly captured by Communists, and was never heard from again. My mom was glad to be rid of him, and told me that he was very lucky to have escaped an even worse fate at her hands.
It's funny how my life has to a large degree paralleled hers in certain ways, despite living in a very different country now. We never look for trouble, but it always manages to find us anyway, and all we can do is become more vicious and savage than our enemies whenever we're forced to.
Anyway, as for visual deterrents, since security signs don't seem to be particularly effective, at least from what I've seen, perhaps the following would make some burglars look elsewhere: